Process for annealing austenitic steels



Patented Oct. 27', 1942 PROCESS FOR ANNEALING AUSTENITIC STEEIJS Otto Dahl, Berlin Wilmersdorf, and Franz Pawlek, Berlin-Kopenick, Germany, assignors to General Electric Company, a corporation of New York No Drawing. Application June 20, 1940, Serial No. 341,596. In Germany June 27, 1939 2 Claims.

The present invention relates to an annealing process; and more particularly to a process for annealing austenitic chromium-nickel steels in a nitrogen-hydrogen mixture. Austenitic' chromium-nickel alloys absorb nitrogen by diffusion durmg the annealing in ammonia or hydrogen-nitrogen mixtures, which may be obtained by the decomposition and burning of ammonia. At the high annealing temperature, the nitrogen is bound by the austenite in a solid solution, and contributes to its stabilization.

Nitrogen-hydrogen mixtures have been employed in bright annealing, especially the bright annealing of colored metals, such as copper and its alloys, for instance brass. For ordinary steel,

- this annealing atmosphere is generally too expensive.

The fact that nitrogen-hydrogen mixtures can be applied, especially for the bright annealing of austenitic chromium-nickel steel, etc. was not known heretofore. Up to the present, austenitic chromium-nickel steels were first annealed in a' normal oxidizing atmosphere, and the scale which entirely destroys the rust-resisting qualities, was carefully eliminated by pickling. Attempts have been made to anneal the alloys in thoroughly purified hydrogen, but this practice has not found favor since the hydrogen consumption renders TABLE I Annealing tests in various atmospheres at 1100 C.

Cortoded in Untreated CHSOFHSO4 Annealing atmosphere Coohng Tensile Elonga- Tensile Elongastrength tion strength tron KgJm'm. Percent K p/min. Percent Rapid--. 63 4s 63 4s {Slow 51 1g 7 43 70 Hydrogen {gg a 73 40 7 43 90 48 90 1 Split ammonia {g zgi 93 40 9 0 TABLE II Annealing tests in dissociated ammonia at Time of annealing in minutes $335? 685 Per cent the process very expensive, and moreover it is necessary to quench the alloys after annealing in the case of a carbon content of more than 0.06 per cent). In the case of alloys without carbon, or having an additional amount of carbide-forming substances, the quenching is assumed to be advantageous. The reason that neither nitro genhydrogen mixtures nor ammonia, were not used heretofore for the bright annealing of austenitic chromium-nickel steels mustbe attributed to the belief that the absorption of nitrogen would eliminate the resistance to rust.

A chromium nickel steel with the usual content of 18 per cent chromium and 8 per cent nickel of normal manufacture, that is to say, with carbon content exceeding 0.06 per cent, was used in the form of thin bands of 0.2 mm. thickness. This band was heated in a furnace into which dissociated ammonia was introduced. It was first heated up to 1100 C. for one-half hour, after which it was cooled, first slowly (within 5 to 6 hours) and then rather rapidly, by shifting it into the cold zone of the furnace, so that the rate of cooling would correspond to that obtainable in air. The same annealing was carried out in a hydrogen atmosphere and also in air. The specimens which were annealed in the nitrogen-hydrogen mixture by far exceed the other specimens,

. specimens, inter-crystalline corrosion reduced the elongation to zero. However, in the specimens that were more rapidly cooled, the original since the pickling which scaling requires can be dispensed with.

The increase in strength must be attributed to the nitrogen absorption, and requires, owing to the diffusion process, a certain amount of time, so that the period of heating must each time be adapted to the thickness of the material which is strength was entirely retained, and the gain in strength caused by the nitrogen absorption was also maintained. The same results were obtained with rusting tests that were conducted in an air atmosphere and in a salt-spray fog. Also in this case, the resistivity to rust is reduced only in the slowly cooled specimens. The reduced resistivity to corrosion of the slowly cooled specimens can beattributed only to the carbon, and is therefore equal in all specimens. Inasmuch as the increase in strength must be attributed to the nitrogen increase of the austenite crystals, it is selfevident (and it is corroborated by further tests) that this increase will occur not only in carbon steels of normal manufacture, but also in special steels, having a low carbon content and additions of carbide-forming agents. Under this condition, there will be no difference between a slowly tures made of dissociated ammonia lies also in the fact that owing to the absence of oxygen and to .the ease with which water vapor can be eliminated, scaling or coloring of the annealed material is prevented. The specimens mentioned in the preceding paragraphs, whichhad been subjected to the rust and corrosion test, had not been pickled prior to the tests. On the contrary, they were subjected to the corrosion test just as they came out of the furnace.

It will be seen from the preceding thatthe annealing in nitrogen-hydrogen mixtures, which mixtures can be obtained by the decomposition of ammonia or by decomposition and burning, brings. about a considerable increase in strength and thus a desirable improvement of the mechanical properties. At the same time, the surface is protected, since no scaling occurs, and

to be annealed. This is in contrast to the normal practice observed in soft annealing. For instance, as indicated in Table II a wire of 1 mm. thickness shows, after a short annealing period of 5 minutes, such as would suffice for the soft annealing in a continuous heating furnace, does not disclose any increase in strength. Only after three hours is the strength increased. In general, this nitrogen will be applied for, absolute permeation, and in this manner the strength can be increased throughout the entire cross section, but in certain cases, for instance when it is desired to increase the fatigue strength of thicker construction parts, it will sufilce to limit the nitriding to the external zones. In this manner, the same improvement is obtained which would be possible by the strengthening of edge layers according to other methods.

This method is not limited to chromium-nickel steels of the composition stated in these specifications,.of 18 per cent chromium and 8 per cent nickel, but it can be applied to all austenitic chromium-nickel alloys.

-What we claim as new and desire to secure by Letters Patent ofthe United States, is:

1. The process for annealing austenitic chromium-nickel alloy steel which comprises heating said steel in a furnace and in a nitrogen-hydrogen atmosphere at a temperature of about 1100 C.-for about one-half hour, slowly cooling the steel in the furnace for about five to six hours and thereafter quickly cooling it.

2. The process for annealing austenitic chromium-nickelalloy steel which comprises heating.

half hour, cooling the steel at a rate. corresponding to that obtainable in air for about five to six hours and thereafter quickly cooling the steel.

o'r'ro DAHL. FRANZ PAWLEK. 

